10.1.1 Stra tegy 1- Large Volume s of Material.
Fatigue testing of large volumes of material such that a significant number of fractures from inclusions are generated would be the simplest approach. However, in practice, this would be very expensive since, as noted above, even large disks do not always fracture from inclusions. Tests should also be conducted at a wide range of temperatures and load levels to ensure that the material behavior has been characterized appropriately through the full-service usage envelope.
10.1.2 Strategy 2 - Seeded Material .
Fatigue testing of seeded material where artificial inclusions of a known size and rate are introduced at the powder stage is another approach. The seeds should be representative of the actual inclusions in composition and behavior. Specimens and components manufactured from seeded material are much more likely to fail from the artificial seeds than through conventional LCF. This allows a fatigue model for the inclusions to be generated based on a relatively modest test program.
10.2 Defining Equivalent Crack Size.
10.2.1 Regardless of how the applicant performs the tests, the applicant should define an equivalent crack size that corresponds to the end of the nucleation phase of life and the start of crack growth. The applicant could show equivalency in various ways, for example:
• Engineering crack size (0.030” x .015” for surface cracks, but less easily defined for sub-surface cracks);
• The actual size of the anomaly from which the fatigue failure nucleated (determined by fractography); or
• The actual size of the anomaly from which the fatigue failure nucleated, plus an allowance for diffusion or other interaction between the anomaly and the surrounding material.
10.2.2 Determining the nucleation life is likely to involve back calculations using fracture mechanics of the number of cycles from either test piece, failure, or crack detection (of a known size) to the equivalent crack size at the end of the nucleation life.
10.2.3 For crack nucleation from porosity, the applicant should show equivalency as described in paragraph 10.2.1. It may be possible to show that the size and frequency distribution for porosity can be determined directly from conventional specimens because pores are often more frequent than inclusions and have a lower scatter in size. The applicant may also consider seeding studies, using methods such as atomizing powder at off-nominal conditions to generate porosity at higher rates than at the nominal conditions. The stress component (von Mises or worst principal, for example) that best describes crack nucleation needs to also be defined.
Note: The stress component may be different for a pore, an inclusion, or a grain facet.
8/15/23 11 CALIBRATION AND VALIDATION OF THE PROBABILISTIC SYSTEM FOR INCLUSIONS.
AC 33.70-4
11.1 As a probabilistic system integrates a significant number of parameters (related to the size distribution and life model) that are often identified independently, applicants should verify that the probabilistic sy stem produces sensible results once these are combined. As part of achieving this, the inclusion size distribution may be slightly adjusted in the domain that is too small to be easily characterized experimentally.
11.2 The confirmation of the accuracy of the probabilistic system for inclusions can be made using fatigue test results from specimens or components made from unseeded material.
Although only a fractio n will fracture from inclusions, applicants should ensure that these occurrences are sorted to correlate the various probabilities that the probabilistic system can calculate. For example, applicants should consider the following:
• Conditional probabilities of failure from surface, sub-surface, and internal inclusions;
• Probabilities of failure from inclusions within a given size interval; and
• The global failure probability distribution from inclusions as a function of cycles at a given loading condition.
11.3 It is important that applicants run a test campaign to show the accuracy or at least the conservatism of the probabilistic system. In addition to fatigue specimen tests, this should include tests that repres ent the behavior of components. Spin tests are useful in such campaigns to provide information on how to account for component characteristics, such as volume of material under stress, multiaxial loading, and residual stresses.
11.4 The results from the component-representative tests should fall within the population of predicted lives at the test conditions.
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